This invention relates generally to systems for transmitting a signal output from an electromechanical sensor to a location where the signal may be processed to determine value for a physical parameter detected by the sensor. This invention relates particularly to converting an electrical signal output from an electromechanical sensor into an optical signal for transmission to a processing location remote from the electromechanical sensor. Still more particularly, this invention relates to a system for applying an electrical signal output from an electromechanical sensor to an optical phase modulator included in a leg of an interferometer to produce an optical phase shift that is a function of the electrical signal.
Electromechanical sensors such as geophones are planted in the earth to detect ground motion. Applications of geophones include geophysical exploration and earthquake detection and measurement. Moving mass geophones that use the Faraday effect to detect ground motion are well-known in the art.
Other electromechanical sensors such as piezoelectric hydrophones are placed in underwater cables to detect acoustic waves in the ocean. A piezoelectric hydrophone responds to water pressure changes in the acoustic waves to generate a voltage.
There are examples of electrical-mechanical sensors that exist in systems requiring hundreds, and in some cases thousands of sensors, that are used to generate acoustic maps or signatures covering a large spatial volume. These maps can be used to detect subterranean oil deposits and shipping traffic for example. Such sensors are also used in ocean bottom cables and in boreholes.
The outputs of all of these sensors are electrical in nature and must be sent back to a remote location for data retrieval. Coaxial cables or twisted pair cables with multiplexing capability and with limited bandwidth and range are typically used. In a conventional system, active signal amplification of each sensor output at or very near the sensor location is necessary to obtain a sufficiently large output at the remote location for obtaining any useful data.
The present invention overcomes the disadvantages of conventional electromechanical systems by converting the electrical signal output from each sensor into an optical signal that can be transmitted on a fiber optic telemetry cable without amplification. The present invention also allows filtering of the electrical signal before it is converted into an optical signal.
A sensor system according to the present invention comprises an electromechanical sensor that produces an electrical signal in response to a change in a physical parameter being monitored by the sensor. An optical phase modulator is connected to the electromechanical sensor to receive the electrical signal therefrom. An optical interferometer is arranged to have the phase modulator included in one its legs such that the electrical signal output by the electromechanical sensor causes the phase modulator to produce a phase change between optical signals propagating in the first and second legs. The phase change is related to the magnitude of the change in the physical parameter.
Preferably both legs of the interferometer include an optical phase modulator connected in push-pull to increase the phase change and provide a stronger optical signal.
The sensor system also preferably includes a filter connected between the output of the electromechanical sensor and the phase modulators so that only voltages associated with a selected frequency range are applied to the phase modulators.
An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings.